When checking the condition of broiler breeders, it is common practice to look at the formation of their breast muscle. There are several scoring systems available, but it is generally accepted that, especially for high yielding lines, the breast muscle should have a sort of U shape if you consider the keel bone as the deepest point of that shape. A V shape will mean that the bird is not developed enough, a W shape will mean that the bird is overfleshed.
However, also the firmness of the breast muscle can be an indicator for the condition of the birds, especially after peak production. When the birds are losing condition for instance because the feed supply is not sufficient, we often can feel that the breast muscle becomes a bit more weak and is not that solid and rigid anymore, even before the muscle starts to shrinken.
We can check that by feeling the breast muscle and comparing it with the tenderness of a grilled steak. There is a method to evaluate the level of tenderness on a steak. If you have your hand open and you press the fleshy part of your hand under the thumb, it resembles the tenderness of a raw, not cooked steak. If you touch with your index finger the tip of your thumb (so you make the "ok" signal), the fleshy part of your hand under the thumb resembles a "rare" cooked steak. With your middle finger on the top of your thumb it feels like "medium rare", ring finger on your thumb feels like "medium" cooked and pinky finger on your thumb feels like "well done".
If you feel the breast muscle of a mature broiler breeder, it doesnt feel hard like a stone, but it should have the structure that resembles a "well done" steak. If you repeat this observations regulary and you feel that the firmness of the breast muscle is moving from "well done" to "rare", it might be that the birds are starting to lose condition.